An optical security device and method of manufacture

ABSTRACT

A security device is disclosed including a substrate and one or more focusing elements or lens structures located on one side of the substrate. The security device includes a plurality of image elements associated with each focusing element wherein the image elements include at least first and second groups of image elements. Each image element may be composed of pixels located in an object plane to be viewable through the associated focusing element. Each image element comprises a diffractive grating element or sub-wavelength grating element which when illuminated by a light source generates a diffraction image observable at a range of viewing angles around the device. Image elements of the first group are visible in a first range of viewing angles and image elements of the second group are visible in a second range of viewing angles. The security device is particularly suitable for use on security documents, such as banknotes. A method of forming a security device is also disclosed.

FIELD OF THE INVENTION

The present disclosure relates to optically variable devices and methodsfor their manufacture and/or verification. In particular the presentdisclosure relates to optical devices which include diffractive elementsor structures in their construction. More specifically, the presentdisclosure relates to optical devices which generate an opticallyvariable effect including one or more colour images, and/ormonochromatic or grayscale images.

BACKGROUND TO THE INVENTION

Optical security devices are commonly used in connection with valuabledocuments as a means of avoiding unauthorised duplication or forgery.These security devices typically produce optical effects and/or featureswhich may be difficult for a potential counterfeiter to replicate. Theoptical effects and/or features may also be used for verification of thevaluable documents.

Counterfeiting of banknotes and other valuable documents has become anincreasingly important issue in recent times due to ready availabilityof color photocopiers and computer scanning equipment. This technologyprovides counterfeiters with an easier route to copying of valuabledocuments issued using traditional security printing technologies. Inresponse, central banks and banknote printers have turned totechnologies and devices which produce images that vary with changingangle of view, and which therefore cannot be easily photographed.

Such devices known collectively as optically variable devices (OVDs),have successfully reduced the incidence of counterfeiting using computerscanning equipment. OVD technologies are able to generate a range ofoptical effects including moving guilloche and graphic effects. OVDtechnologies include dot matrix hologram technology (EP 0 467 601 A2),KINEGRAM™ technology (EP 105099, EP 330 738, EP 375 833) first used onSaudi Arabian passports in 1987 and later on Austrian 5000 Schillingbanknotes in 1990, CATPIX™ grating technology (PCT/AU89/00542) used onAustralian plastics ten dollar banknotes issued in 1988 and Singaporeplastics 50 dollar banknotes in 1990, PIXELGRAM™ technology (U.S. Pat.No. 5,428,479), and EXELGRAM™ technology (PCT AU/94/00441) first used onAustralian opal stamps and Vietnam bank cheques issued in 1995.

PIXELGRAM™ and EXELGRAM™ technologies display relatively high resolutionportraiture effects that change from positive tone to negative toneimages as the angle of view is changed. Printed high resolutionportraiture has long been used on banknotes as a security featurebecause of the human eye's ability to perceive errors or defects in animage of a human face. Accordingly PIXELGRAM™ and EXELGRAM™ technologieswere developed to include portraiture OVD effects. However, theseportraiture effects were limited to near monochromatic images consistingof a fixed number of brightness levels or “greyness” values. PCTapplication PCT/AU97/00800, published under WO98/23979 entitled ColorImage Diffractive Device, the disclosure of which is herein incorporatedby cross reference, extended diffractive OVD imagery to generate fullcolor images.

However, there have been limitations and difficulties in displayingdiffractive OVD imagery on physical substrates. In particular, surfacerelief holograms and holographic stereograms viewable under white lightsuffer blurring and a loss of clarity when viewed with extended (notpoint) light sources. Regions of the image furthest from the plane ofthe hologram suffer the worst. This occurs because while verticalparallax has been eliminated to prevent spectral smear, horizontalperspectives which determine dimensionality are sent in incorrectdirections by a light source which is even partly diffuse in thehorizontal direction. This problem increases under extended or diffuselight sources and with distance above or below the image plane.

Various technologies have been used to create three dimensional images,some of which are based on printing combined with lenticular optics. Todate print versions are limited to relatively low resolution.Holographic stereograms based on diffractive OVD imagery are capable ofmuch higher resolution, although without the use of special lightingthey generally have limitations as described above.

Holographic stereograms have been produced by Pacific Holographics Inc.and others in the past and one remedy was to compromise by reducingimage size and depth. Despite these constraints the results weredifficult to view in typical lighting environments.

Hence there is a need to increase security of valuable documents such aspolymer banknotes using OVD technology incorporating 3D imagery. TheseOVD's may use focusing elements such as micro lens arrays, and mayincorporate diffractive devices. There is also a need to address thelimitations of displaying 3D imagery on physical substrates based onsuch diffractive devices.

SUMMARY OF THE INVENTION

In broad terms in a first aspect, there is provided a security devicecomprising:

-   -   a plurality of focusing elements;    -   a plurality of image elements associated with each focusing        element wherein said image elements include at least a first and        a second group of image elements,        -   wherein each image element is located in an object plane to            be viewable through the associated focusing element,        -   wherein each image element comprises a diffractive grating            element or sub-wavelength grating element which when            illuminated by a light source generates a diffraction image            observable at a range of viewing angles around the device,            and image elements of the first group are visible in a first            range of viewing angles and image elements of the second            group are visible in a second range of viewing angles.

In one form, the image elements include three or more groups of imageelements to represent an image observable from different viewing angles.

In one form, the diffraction image is a greyscale or monochromatic imagethat includes a plurality of brightness levels across the image, and/ora color image that includes a plurality of colours.

In one form, each image element comprises red, green and bluesub-elements.

In one form, the red, green and blue sub-elements each includes its owndiffractive grating element or sub-wavelength grating element, whereinthe frequency and/or the pitch of the diffractive grating elements orsub-wavelength grating elements are different in the red, green and bluesub-elements so that each sub-element produces a predetermined primarycolour upon illumination.

In one form, the red, green and blue sub-elements are verticallyarranged as a strip.

In one form, the red sub-element located at the top of a vertical stripof grating elements, the green sub-element located in the middle of thestrip, and the blue sub-element located at the bottom of the strip.

In one form, the red, green and blue sub-elements are of a same physicalsize.

In one form, the grating elements of the red, green and bluesub-elements have a size distribution and/or a spatial distributioncorresponding to grey levels or brightness levels associated with thesub-element.

In one form, each of the sub-elements includes an effective grating areathat includes the diffraction grating element or the sub-wavelengthgrating element, and a non-diffractive area that does not include anygrating elements, and a brightness value of each sub-element is variedby changing the effective grating area within each sub-element and/orthe non-diffractive area of the sub-element.

In one form, the non-diffractive area within each of the red, green andblue sub-elements are of a same size for a given image element, tothereby generate a greyscale image, or that the non-diffractive areawithin each of the red, green and blue sub-elements are of a differentsize for a given image element, to thereby generate a colour image.

In one form, each image element includes a randomized diffractiongrating with a randomized grating pitch and/or width to diffractincident light at different angles, such that incident light diffractedfrom the diffraction grating is diffused and the diffraction imageobservable is a greyscale image.

In one form, each image element includes an effective grating area thatincludes the diffraction grating element or the sub wavelength gratingelement, and a non-diffractive area that does not include any gratingelement, and a brightness value of each image element is varied bychanging the effective grating area and/or the non-diffractive area ofthe image element.

In one form, the non-diffractive area includes a microstructured surfacewith light traps for creating internal reflections of most incidentlight that then fails to reflect out and away from the light traps.

In one form, the non-diffractive area has a reflective coating forspecular reflection of incident light.

In one form, the non-diffractive area has a light absorbing coating.

In one form, the image elements are anamorphic, and more preferably theimage elements are configured to have a rectangular shape, meaning thatthe lengths of the image elements are greater than the widths.

In one form, the resolution of the image elements are 12 times, or 10times, or 8 times, or 4 times higher vertically than horizontally.

In one form, the diffractive grating element or sub-wavelength gratingelement is formed from a surface relief structure.

In one form, the image elements are formed from a radiation curable ink.

In one form, wherein the focusing elements are formed from a radiationcurable ink by printing and/or embossing.

In one form, the focusing elements include refractive or diffractivepart-cylindrical lenses or zone plates.

In one form, the diffraction image observable is a three dimensionalimage of a scene, object and/or a person

In broad terms in a second aspect, there is provided a security devicecomprising:

-   -   a plurality of focusing elements;    -   a plurality of image elements located in an object plane to be        viewable through the focusing elements, said image elements        including at least first and second groups of image elements,        -   wherein each image element includes a surface structured for            causing diffuse scattering of incident light, wherein said            plurality of image elements are arranged to generate an            image observable when illuminated by the incident light, and            image elements of the first group are observable in a first            range of viewing angles and image elements of the second            group are observable in a second range of viewing angles.

In one form, the surface structured for causing diffuse scattering ofincident light includes a randomized diffraction grating with a randomgrating pitch to diffract light of a given wavelength at differentangles such that white incident light diffracted from the randomdiffractive grating is diffused and the image observable is a greyscaleimage.

In one form, the surface structured for causing diffuse scattering ofincident light may include an array of randomly arranged micromirrorswhich cause incident light to be diffuse scattered in differentdirections.

In one form, each image element includes an effective grating area thatincludes the randomized diffraction grating element, and anon-diffractive area that does not include any grating element, and abrightness value of each image element is varied by changing theeffective grating area and/or the non-diffractive area of the imageelement.

In one form, the non-diffractive area includes any one or more of:

-   -   a microstructured surface with light traps for creating internal        reflections of most incident light that then fails to reflect        out and away from the light traps,    -   a reflective coating for specular reflection of incident light,        and    -   a light absorbing coating.

In one form, the image elements are anamorphic, and more preferably theimage elements are configured to have a rectangular shape, meaning thatthe lengths of the image elements are greater than the widths.

In one form, wherein the resolution of the image elements are 12 times,or 10 times, or 8 times, or 4 times higher vertically than horizontally.

Preferably, the image observable is a three dimensional image of ascene, object and/or a person.

In a third aspect, there is provided a method of forming a securitydevice including the steps of:

-   -   providing a substrate;    -   applying a plurality of focusing elements to a first surface of        the substrate; and    -   applying a plurality of image elements to an image surface of        the substrate including at least a first group of image elements        and a second group of image elements, wherein each image element        is located in an object plane to be viewable through an        associated focusing element,    -   wherein each image element comprises a diffractive grating        element or sub-wavelength grating element which when illuminated        by a light source generates a diffraction image observable at a        range of viewing angles around the device; and wherein image        elements of the first group are visible in a first range of        viewing angles and image elements of the second group are        visible in a second range of viewing angles.

In one form, the method further includes the step of applying a layer ofembossable radiation curable ink to the substrate prior to beingembossed while soft and curing the ink by radiation to form the one ormore focusing elements on one side of the substrate.

In one form, the diffraction image includes a greyscale image, and/or amulti-colour image.

In a fourth aspect, there is provided a security document, including asecurity device according to either the first or the second aspects, ora security device manufactured according to the third asepct.

In one form, the security element or security device is provided withina window or half-window region of the security document.

In one form, the security document includes a banknote, passport, creditcard or cheque.

In one example the image may include a portrait of an object such as ahuman face and groups of image elements or channels may represent theobject from many different viewpoints. Projectional views of the objectmay be captured so that the final stereogram produces an accuratethree-dimensional image of that object. One technique for capturingstereograms is described in an MIT paper entitled “The GeneralizedHolographic Stereogram” by Michael W. Halle:

http://www.media.mit.edu/spi/SPIPapers/halazar/halle91.pdf the contentsof which are incorporated herein by reference.

According to the present disclosure, diffractive devices (classical ordirect write) and multiple channels are used in place of ink to createmuch higher resolution images. This allows use of microscopic lenticularstructures necessary for currency, high resolution imagery, and “true”colored images. In addition, hybrid 2D/3D computer graphics may beintegrated with photographic stereograms.

The one or more aspects of the present disclosure were not previouslyavailable because manufacturing an inexpensive clear substrate withmicro-lenses was relatively unknown. In any event previous use ofmultiplexed images with lenticular lenses to form three dimensional aswell as animated “Flip” images has historically been limited to printedimage elements, which generally only include printed lines or dots.

However, print forms of this technology suffer from resolutionlimitations due to the nature of ink and the narrow channels requiredmultiplexing the images under lenticular lenses. This requires eitherthicker and lower resolution lenticular lenses or very crude black andwhite imagery. 3D Lenticular images created in ink are crude due toresolution limits of commercial printing as this may limit the number ofchannels, or angular views.

Applicant is not aware of any previous attempt to combine focusingelements for example cylindrical micro-lenses with diffractivestereograms. Diffractive stereograms without the benefit of additionaloptical elements suffer from limitations as described above.

Any reference herein to a patent document or other matter which is givenas prior art is not to be taken as an admission that that document ormatter contains information which was part of the common generalknowledge at the priority date of any of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described byway of example and with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a pixel of an optically variablediffractive device including three primary color sub-pixels.

FIG. 2 is a schematic diagram of the manner in which a diffractivesurface relief structure diffracts incident light.

FIG. 3 is a schematic diagram of a mechanism of a full color diffractivedevice.

FIG. 4 shows one method of controlling brightness of primary colorsub-pixels.

FIGS. 5A and 5B show another method of controlling brightness of primarycolor sub-pixels.

FIG. 6 shows an example of an eight-channel OVD pixel structure.

FIGS. 7A and 7B are schematic diagrams of two different forms of pixelfor generating a grayscale image.

FIGS. 8A and 8B are schematic diagrams of the operation of the grayscalepixel shown in FIG. 7B.

FIG. 9 shows a cross sectional view of an optical security deviceincorporating an eight-channel OVD structure according to one embodimentof the present disclosure.

FIG. 10 shows a cross sectional view of an optical security deviceaccording to another embodiment of the present disclosure.

FIG. 11 shows a flow chart of a production process for an opticalsecurity device according to an embodiment of the present invention.

FIG. 12 shows a flow chart of a diffractive structure manufactureprocess associated with an optical security device according to anembodiment of the present invention.

FIG. 13 shows three different methods appropriate for generating imagesaccording to the present invention.

FIG. 14 shows how a device of the present disclosure is viewed by leftand right eyes of a viewer.

DESCRIPTION OF PREFERRED EMBODIMENTS DEFINITIONS

Security document

As used herein, the term security document includes all types ofdocuments and tokens of value and identification documents including,but not limited to the following: items of currency such as banknotesand coins, credit cards, cheques, passports, identity cards, securitiesand share certificates, driver's licences, deeds of title, traveldocuments such as airline and train tickets, entrance cards and tickets,birth, death and marriage certificates, and academic transcripts.

The invention is particularly, but not exclusively, applicable tosecurity documents or tokens, such as banknotes, or identificationdocuments, such as identity cards or passports, formed from a substrateto which one or more layers of printing are applied.

The diffraction gratings and optically variable devices described hereincan also have application in other products, such as packaging.

Security Device or Feature

As used herein the term security device or feature includes any one of alarge number of security devices, elements or features intended toprotect the security document or token from counterfeiting, copying, andalteration or tampering. Security devices or features can be provided inor on the substrate of the security document or in or on one or morelayers applied to the base substrate, and can take a wide variety offorms, such as security threads embedded in layers of the securitydocument; security inks such as fluorescent, luminescent andphosphorescent inks, metallic inks, iridescent inks, photochromic,thermochromic, hydrochromic or piezochromic inks; printed and embossedfeatures, including relief structures; interference layers; liquidcrystal devices; lenses and lenticular structures; optically variabledevices (OVDs) comprising reflective optical structures includingreflecting surface relief structures and diffractive devices includingdiffraction gratings, holograms and diffractive optical elements (DOEs).

Transparent Windows and Half Windows

As used herein the term window refers to a transparent or translucentarea in the security document compared to the substantially opaqueregion to which printing is applied. The window may be fully transparentso that it allows the transmission of light substantially unaffected, orit may be partly transparent or translucent partially allowing thetransmission of light but without allowing objects to be seen clearlythrough the window area.

A window area may be formed in a polymeric security document which hasat least one layer of transparent polymeric material and one or moreopacifying layers applied to at least one side of a transparentpolymeric substrate, by omitting least one opacifying layer in theregion forming the window area. If opacifying layers are applied to bothsides of a transparent substrate a fully transparent window may beformed by omitting the opacifying layers on both sides of thetransparent substrate in the window area.

A partly transparent or translucent area, hereinafter referred to as a“half-window”, may be formed in a polymeric security document which hasopacifying layers on both sides by omitting the opacifying layers on oneside only of the security document in the window area so that the“half-window” is not fully transparent, but allows some light to passthrough without allowing objects to be viewed clearly through thehalf-window.

Alternatively, it is possible for the substrates to be formed from ansubstantially opaque material, such as paper or fibrous material, withan insert of transparent plastics material inserted into a cut-out, orrecess in the paper or fibrous substrate to form a transparent window ora translucent half-window area.

Opacifying Layers

One or more opacifying layers may be applied to a transparent substrateto increase the opacity of the security document. An opacifying layer issuch that L_(T)<L_(O), where L_(O) is the amount of light incident onthe document, and L_(T) is the amount of light transmitted through thedocument. An opacifying layer may comprise any one or more of a varietyof opacifying coatings. For example, the opacifying coatings maycomprise a pigment, such as titanium dioxide, dispersed within a binderor carrier of heat-activated cross-linkable polymeric material.Alternatively, a substrate of transparent plastic material could besandwiched between opacifying layers of paper or other partially orsubstantially opaque material to which indicia may be subsequentlyprinted or otherwise applied.

Embossable Radiation Curable Ink

The term embossable radiation curable ink used herein refers to any ink,lacquer or other coating which may be applied to the substrate in aprinting process, and which can be embossed while soft to form a reliefstructure and cured by radiation to fix the embossed relief structure.The curing process does not take place before the radiation curable inkis embossed, but it is possible for the curing process to take placeeither after embossing or at substantially the same time as theembossing step. The radiation curable ink is preferably curable byultraviolet (UV) radiation. Alternatively, the radiation curable ink maybe cured by other forms of radiation, such as electron beams or X-rays.

The radiation curable ink is, preferably, a transparent or translucentink formed from a clear resin material. Such a transparent ortranslucent ink is particularly suitable for printing light-transmissivesecurity elements, such as sub-wavelength gratings, transmissivediffractive gratings and lens structures.

In one particularly preferred embodiment, the transparent or translucentink preferably comprises an acrylic based UV curable clear embossablelacquer or coating.

Such UV curable lacquers can be obtained from various manufacturers,including Kingfisher Ink Limited, product ultraviolet type UVF-203 orsimilar. Alternatively, the radiation curable embossable coatings may bebased on other compounds, e.g. nitro-cellulose.

The radiation curable inks and lacquers used herein have been found tobe particularly suitable for embossing microstructures, includingdiffractive structures such as diffraction gratings and holograms, andmicro lenses and lens arrays. However, they may also be embossed withlarger relief structures, such as non-diffractive optically variabledevices.

The ink is preferably embossed and cured by ultraviolet (UV) radiationat substantially the same time. In a particularly preferred embodiment,the radiation curable ink is applied and embossed at substantially thesame time in a Gravure printing process.

Focal Point Size H

As used herein, the term focal point size refers to the dimensions,usually an effective diameter or width, of the geometrical distributionof points at which rays refracted through a lens intersect with anobject plane at a particular viewing angle. The focal point size may beinferred from theoretical calculations, ray tracing simulations, or fromactual measurements.

Focal Length f

In the present specification, focal length, when used in reference to amicro lens in a lens array, means the distance from the vertex of themicro lens to the position of the focus given by locating the maximum ofthe power density distribution when collimated radiation is incidentfrom the lens side of the array (see Miyashita, “Standardization formicro lenses and micro lens arrays” (2007) Japanese Journal of AppliedPhysics 46, p 5391).

Gauge Thickness t

The gauge thickness is the distance from the apex of a lenslet on oneside of the transparent or translucent material to the surface on theopposite side of the translucent material on which the image elementsare provided which substantially coincides with the object plane.

Lens Frequency and Pitch

The lens frequency of a lens array is the number of lenslets in a givendistance across the surface of the lens array. The pitch is the distancefrom the apex of one lenslet to the apex of the adjacent lenslet. In auniform lens array, the pitch has an inverse relationship to the lensfrequency.

Lens Width W

The width of a lenslet in a micro lens array is the distance from oneedge of the lenslet to the opposite edge of the lenslet. In a lens arraywith hemispherical or semi-cylindrical lenslets, the width will be equalto the diameter of the lenslets.

Radius of Curvature R

The radius of curvature of a lenslet is the distance from a point on thesurface of the lens to a point at which the normal to the lens surfaceintersects a line extending perpendicularly through the apex of thelenslet (the lens axis).

Sag Height s

The sag height or surface sag s of a lenslet is the distance from theapex to a point on the axis intersected by the shortest line from theedge of a lenslet extending perpendicularly through the axis.

Refractive Index n

The refractive index of a medium n is the ratio of the speed of light invacuum to the speed of light in the medium. The refractive index n of alens determines the amount by which light rays reaching the lens surfacewill be refracted, according to Snell's law:

η* Sin(a)=n*Sin(θ), where a is the angle between an incident ray and thenormal at the point of incidence at the lens surface, θ is the anglebetween the refracted ray and the normal at the point of incidence, andη is the refractive index of air (as an approximation η may be taken tobe 1).

Lobe Angle

The lobe angle of a lens is the entire viewing angle formed by the lens.

Abbe Number

The Abbe number of a transparent or translucent material is a measure ofthe dispersion (variation of refractive index with wavelength) of thematerial. An appropriate choice of Abbe number for a lens can help tominimize chromatic aberration.

Sub-Wavelength Grating Element

A sub-wavelength grating element is a grating element having a period orspacing between grating lines that is less than the wavelength, so thatthe dominant diffracted mode is in the zero-order. Effective propertiesmay be independent of the period, as long as it is sufficientlysub-wavelength. Properties may also be tolerant to structuraldeformations.

Image Pixel and Sub Image Pixel

Throughout the description terms such as image element(s) and imagepixel(s) are used interchangeably and they are intended to have the samemeaning. Terms such as sub pixels, sub image pixels, and sub elementsare also intended to have the same meaning as each other and usedinterchangeably.

RGB (Red, Green, Blue) Color Space

A broad range of colors may be constructed with three primary colorsRed, Green and Blue. Any color image may be decomposed into three imagescomprising these primary colors. Each color image may have manyintensity levels of brightness. For example if 16 levels of brightnessare chosen for each primary color, 4,096 different colors may beproduced.

An OVD device may create a portrait in the same way as a video monitormay display a portrait or image on a screen which decomposes the imageinto many elements or pixels. Referring to FIG. 1 of the drawings asingle image element or image pixel 10 is shown which includes red (R),green (G) and blue (B) sub-elements, or sub-pixels, 11, 12, 13 toprovide the three primary colors with different brightness values. Thismechanism may allow a wide range of colors to be reproduced with variousvalues of hue or color and intensity or brightness. If image elementsare produced with primary colors of various brightness, full colorimages may be readily created. RGB is a specific example of oneparticular color space, which is used in embodiments described below,but other color spaces, including grayscale and/or single color systems,are also applicable to all embodiments of the invention.

Referring to FIG. 2 when a diffractive image element 20 with gratingperiod (or spatial frequency) d is illuminated by a collimated whitelight beam in normal incidence, light with different wavelengths, λ (orcolor) is diffracted into different angles, α which are governed by theequation:

$\begin{matrix}{{\sin \; \alpha} = \frac{\lambda}{d}} & (1)\end{matrix}$

Here only the first order of diffraction is considered because mostlight energy is diffracted into the first order. If only single spatialfrequency gratings are fabricated within a pixel of an OVD, the OVDproduces monochromatic images. When a person observes an image on anOVD, there is usually a fixed incidence angle (direction of lightsource) and a fixed angle of observation assuming the observer does notmove too much. Therefore the viewing angle a is relatively fixed.

From equation (1) outgoing diffracted light in various wavelengths orcolors may be achieved by varying d, the period of the gratings. We aremainly interested in the three primary colors Red, Green and Blue havingcorresponding wavelengths λ₁, λ₂ and λ₃. They may be diffracted bygratings in three color sub-elements or sub-pixels having respectiveperiod d₁ d₂ and d₃ as shown in FIG. 3. When the gratings areilluminated by white light, such as a fluorescent tube, at the sameviewing angle a, colored light is diffracted from these sub-pixelsaccording to the equation:

$\begin{matrix}{{\sin \; \alpha} = {\frac{\lambda_{1}}{d_{1}} = {\frac{\lambda_{2}}{d_{2}} = \frac{\lambda_{3}}{d_{3}}}}} & (2)\end{matrix}$

For example, three primary colors with wavelengths λ₁=600 nm, λ₂=500 nmand λ₃=450 nm, and a viewing angle α=30 degrees, correspond to gratingperiods d₁=1200 nm, d₂=1000 nm and d₃=900 nm for each sub-pixel 11-13making up each pixel 10.

It should be appreciated that other combinations of incident angle,viewing angle and grating periods may be chosen other than what isdisclosed above, so long as they satisfy the diffraction equation ofd(sinθ_(m)+sinθ_(i))=mλ, wherein θ₁ is the angle at which the light isincident, θ_(m) is the viewing angle, d is the separation of gratingelements, and m is the diffraction order.

The three primary colors (RGB) are created by gratings in three spatialfrequencies. In order to manipulate hue to achieve full color, thebrightness value of each primary color from the sub-pixels must also becontrolled. There are many methods to control brightness of diffractedlight from regions on the surface of an OVD, such as varying gratingdepth, grating profile and/or grating curvatures. FIGS. 4 and 5illustrate two methods of controlling brightness of RGB sub-pixels11-13.

In FIG. 4 the brightness value of each sub-pixel 11-13 is varied bychanging the effective area of gratings within each sub-pixel. FIG. 4shows a single image pixel comprising RGB sub-pixels 11, 12, 13, whereinthe effective area of gratings of each sub-pixel 11-13 is adjusted inheight or area to represent a particular value of brightness.

In each sub-pixel 11, 12, 13, the brightness of diffracted light isproportional to the area of each diffracting structure 41-43 associatedwith the respective sub-pixel 11-13. In other words the brightness isproportional to the heights or areas of the diffracting structures41-43, as distinct from flat areas 44-49 associated with the sub-pixels11-13 which have an absence of diffracting structures. One advantage ofthis arrangement is that brightness of sub-pixels 11-13 may besubstantially linearly related to the heights or areas of diffractingstructures 41-43.

FIGS. 5(a) and 5(b) illustrates another method of controlling brightnessof each primary color sub-pixel. This is done by using a set of paletteswherein the diffracting structures are oriented in variable directionsto determine various levels of brightness. FIG. 5(a) shows 8 sub-pixels51-58 oriented at 90 degrees corresponding to the highest brightnessvalue within the palette set. FIG. 5(b) shows 8 sub-pixels 51-58oriented at various angles with a corresponding variation in brightnessvalue within the palette set between a highest (90 degrees) and a lowest(45 degrees) brightness value. It is also possible to combine techniquesfor controlling brightness. For example, using a change in orientationof the diffraction grating to compensate for a drop off in brightness ofouter channels due to lens aberrations.

The structures described above may produce a single channel full colorimage wherein a “single channel” image is the only diffractive imageproduced by the surface relief structure. Multi-channel OVD's (that is,diffractive devices which generate more than one diffraction image) aredesired for stereoscopic applications wherein different images areobservable from different viewing angles. For example “X” number ofchannels may be required to observe stereoscopic portraiture, whereineach channel represents a different viewing angle associated with theportraiture.

Spatially divided space may be one way to achieve multi-channel OVDdevices. A notional element or pixel region on the device may be dividedinto a plurality number of channels wherein each channel contains threecolor sub-elements or sub-pixels for red, green and blue.

FIG. 6 shows an example of a RGB element or pixel structure 60 for aneight-channel full color OVD. RGB pixel structure 60 includes anamorphicimage pixels 61-68, with each entire image pixel 61-68 being representedby a trio of RGB sub-pixels 61R-68R, 61G-68G, 61B-68B arrangedvertically in strips. In the example given, each image pixel 61-68 has avertical dimension of 96 μm and a horizontal dimension of approximately8 μm, such that the resolution of each image pixel 61-68 is 12 timeshigher horizontally than vertically.

It should be appreciated that the physical dimensions of the imagepixels are generally determined by the number of image channelsunderneath each lens, and the pitch of the focusing elements such aslenticular lenses. The horizontal dimension of the image pixels can beanywhere between 4 and 10 microns, and the vertical dimension of theimage pixels can be in the range of 45 to 100 microns. The number ofimage channels can be anywhere between 2 image channels to 16 imagechannels.

It should also be appreciated that the projected diffractive imageryobservable by a viewer is determined by the image content stored in eachimage channel. For example, if the image content stored in each imagechannel is closely associated with the image content stored in otherimage channels, with each image channel representing a view of an objectfrom a certain viewing angle, the device can be configured to project athree dimensional view of that object to the viewer. It is also possibleto achieve other types of diffractive optical effects, such asanimation, morphing, zooming in and/or out, image switching and so on,as the viewing angle changes.

In a most preferred embodiment, the multiple image channels are arrangedto generate a true colour of an image at a predetermined viewing range.The present disclosure offers an OVD which has many advantages over theconventional diffractive or lens structures which include:

-   -   The OVD feature works in a wide range of lighting condition,        including diffuse lighting and low light environment.    -   The feature produces a sharp, well defined image compared with        stereogram images generated by other methods.    -   The OVD produces vibrant diffractive effects and can be        configured to produce true colour mixing effects.    -   The OVD overcomes the limitations associated with traditional        printing technologies and lenticular features.

Extremely high resolution, well beyond the capability of most printers,is required to form the multiple channels, such as 4-channel, 8-channel,10-channel, or even 12-channel under lenticular lenses used on documentssuch as banknotes. As mentioned above, the horizontal resolution of theimage pixels can be as small as 4 microns, whereas with traditionalprinting methods such as gravure and offset printing, due to limitedprinting resolution that inherently exists for any printing method, itis either impossible or extremely difficult to print image elements thathave a resolution of less than 30 to 40 microns.

One means of representing RGB images may yield essentially squaresub-pixels subdivided into vertical columns of RGB wherein the entiresub-pixel would have the same resolution in both directions, althoughRGB sub-pixel divisions require higher resolution.

In the example shown in FIG. 6, the width of each channel is 8 microns,so this would result in sub-pixels 8 microns high, if square sub-pixelswere used. As the sub-pixels are composed of diffractive gratings with arepeat on the order of 1 micron, only 8 grating lines could be recordedin such a sub-pixel. When the height of each diffractive area isadjusted as described above to represent the brightness value of eachimage sub-pixel, this would limit the range of brightness values thatcould be represented. If a broad range of brightness values is to berepresented, more diffractive grating lines must be used. Hence theimage sub-pixels are chosen to be anamorphic, such as each being up to 4times taller than wide.

Greyscale Color Space

In some circumstances, it may be desirable to generate 3D imagerydepicting a greyscale image. One way of generating a greyscale image isto balance the brightness levels between the red, green and bluesubpixels such that the net impression to the viewer, from the portionof the subpixels having diffractive structures, is a mixture of theincoming colors (which for daylight, would be nominally white) and, fromthe entire RGB pixel, a desired greyscale level which together with theremaining pixels generate a greyscale image. This situation is shown inFIG. 7A depicting the diffractive imagery layer beneath a portion of asingle lenticular lens. The imagery layer has a four-channel structurewith two adjacent image pixels shown for each of the four channels.

Referring to channel 1, the adjacent pixels 100 and 102 each have red,green and blue subpixels (111, 112, 113 and 114, 115, 116,respectively). Each of these subpixels has a non-diffractive area usedto vary the brightness level (108, 110, 118, and 119, 120, 121,respectively). In this example, to balance the brightness level betweeneach of the RGB subpixels, the non-diffractive areas 108, 110 and 118are of the same size. Likewise, the non-diffractive area 119, 120 and121 in pixel 102 are also equally sized. As the non-diffractive areas inpixel 100 are smaller than those in pixel 102, the pixel 100 has agreater brightness level than that of 102. This variation in brightnesslevel is used to generate the desired greyscale image. For example, itis clear from FIG. 7A that the upper pixel in channel 2 and the lowerpixel in channel 4 all have the smallest non-diffractive areas andtherefore the highest brightness levels. The greyscale image, in thiscase, is viewable at a specified viewing angle coincident with the1^(st) diffraction order of the RGB subpixels, which are of coursedesigned to be substantially the same, as explained above.

Diffuse White

Alternatively, the 3D greyscale imagery may be generated by pixels witha portion having a surface structure for diffuse scattering of incidentlight and the remaining portion being sized to correspond with apredetermined brightness level. The diffuse scattering may be providedby a surface structure with suitable roughness or, preferably, isprovided by a random diffractive grating structure schematicallyillustrated in FIG. 7B. As with FIG. 7A, FIG. 7B shows the imagery layerbeneath part of a single lenticular lens. Two adjacent image pixels areshown for each of the 4 channels. Referring to channel 4, the adjacentpixels 104 and 106 have a randomized diffractive grating 123 and 125respectively and the non-diffractive areas 122 and 124 respectively. Therandomized (or pseudo-randomized) diffractive gratings 123, 125 eachhave a grating pitch and/or width that varies in a random manner. As theincident light is diffracted from each of the adjacent fringes withinthe grating, the angle of the respective zero order, first order etc.diffractive fringes (lines of positive interference) are generated foreach different wavelength at different angles to the grating. Since thegrating pitch is randomized, so too is this angle of diffraction(importantly, the first order diffraction) for each wavelength ofvisible light, effectively creating a diffuse reflection of the incidentlight. Hence the viewer sees a random or pseudo-random combination ofnumerous different wavelengths which the eye combines as white light. Asdiscussed above, the variance in non-diffractive areas of the diffuselight pixels 104, 106 enable a variance in brightness so that agreyscale image can be generated.

FIGS. 8A and 8B provide a schematic representation of an example of thiseffect. FIG. 8A is a schematic section view of the security device 70.White light 130, 131 is incident on the focusing elements in the form oflenticular lenses 72, 73, 74. Lenticular lenses 72, 73, 74 are supportedon one surface 75 of the transparent substrate 71 while the imageelements 77 and 78 are formed on the opposite surface 76 to provide a 4channel OVD.

The lens geometry is such that parallel incident rays 130 are refractedto a focal strip 132 encompassing the strip of pixels 132 within channel2. Similarly the parallel rays of incident light 131 are focused by lens173 to the strip of pixels 133 in channel 2 of the adjacent imageelements 78. Ideally the width of the focal strips 132 and 133 shouldcorrespond to the width of the pixels, which is achieved via selectionof the appropriate lens geometries and/or substrate thickness.

The light focused onto the focal strips 132 and 133 covering the channel2 pixels is diffracted out of the respective lenses 72 and 73. Atviewing angle α, the diffracted light 134, 135 is that of the greyscalepixels for channel 2, each with a brightness level determined by thesize of their respective non-diffractive surfaces. In this way, thecombination of light from channel 2 pixels 134, 135 etc. combine tocreate a greyscale image seen by the viewer.

Referring to FIG. 8B, a single image element or pixel 136 isschematically shown. Each pixel 136 has a surface area 140 with asurface 142 structured for diffuse diffraction of the incident light. Inthe embodiment of FIG. 8B, the structured surface is a randomized (orpseudo-randomized) diffractive grating in which the grating pitch and/orwidth is random. In one particular form, the UV curable polymer formingthe image layer is structured, using known techniques such as UVembossing or casting, to create a grating of parallel lines with arandom width and a random spacing within a fixed range of widths andspacings.

The incident white light 138 diffracts from the random grating 142 andforms diffraction patterns from each grating line. As the grating pitchis random, so too is the angle of diffraction at which the zero order,first order, second order, and so on, for each wavelength in the whitelight 138. This random dispersion of the zero order, first order, secondorder and so on, diffraction angles 141 means that viewing the pixel 136from viewing angle will see a random collection of wavelengths 143 (λ₁to λ₁). As these wavelengths are drawn from the full spectrum of visiblelight, assuming that the incident light includes the full spectrum ofvisible light, the viewers eye combines the different wavelengths to seewhite light or a close approximation to white light.

The brightness level of the white light 143 seen by the viewer isdetermined by the extent of the non-diffractive area 137. The surface ofthe non-diffractive area 137 may be flat and unstructured such that theincident light 138 will predominately pass through. However, thenon-diffractive surface area may also have a coating such as areflective metallic coating. In this case, the incident light 138 wouldbe specularly reflected at an angle of reflection equal to the angle ofincidence. When viewing the device from that angle, the specularreflection of incident light overpowers the light diffracted from therandomized diffraction grating 142 and the device would appear brightwhite without any discernable greyscale image. At other viewing angleshowever, there would be the necessary contrast between the diffuselydiffracted light from the randomized grating 142 and the non-diffractivesurface area 137 in order to create the greyscale image.

The non-diffractive surface area 137 may also be structured using, forexample UV casting or embossing techniques and/or coated in order toincrease light absorption. Light traps or so called ‘moth-eye’structures, which are described in detail throughout WO2005106601, aresuitable surface microstructures for this purpose. In this case, thenon-diffractive surface area 137 reflects very little of the incidentlight 138 regardless of the incident angle and provides a strongcontrast with the diffusely diffracted light 141 from the randomizedgrating 142. Furthermore, microscopic light trap structures may beembossed into the non-diffractive surface 137 using the same casting orembossing tool that embosses the randomized diffraction grating 142 suchthat both sets of surface structures are in exact register.

As discussed above, in relation to the RGB color space embodiment, thebrightness value of each pixel 136 is varied by changing the size of thenon-diffractive surface area 137. Increasing the size of thenon-diffractive surface area 137 decreases the size of the randomizeddiffractive grating 142 (assuming pixel size 15 constant). Therefore thebrightness level of the light 143 from pixel 136 will also decrease.

Pixels with a surface area structured for diffuse scattering of theincident light to create 3D grayscale images, offer a broader range ofviewing angles in which the grayscale image is seen in the intendeddiffuse color (i.e. white in normal diffuse white incident light). Asdiscussed above, the images generated by the red, green and bluesubpixel embodiments show a true color version of the image at a narrowrange viewing angles only. At other viewing angles, the image is stillvisible but not in true color, instead the image varies according to thediffractive spectrum of the RGB pixels.

A further advantage of the grayscale diffuse pixel embodiments is theimproved image resolution relative to RGB subpixel embodiments. Thecombined area required for the R, G and B sub-pixels, is greater thanthat required for embodiments using a randomized diffraction grating anda single non-diffractive area. Therefore the overall pixel size of thediffuse pixels may be less than one third of the size of the RGB colorpixels. Reducing the pixel size allows the image to have more pixelswhich improves image resolution. That is, whilst the diffuse lightpixels 104, 106 are shown at a size comparative to the entire RGB pixel,they can be the same size as an individual R, G or B subpixel, or evensmaller.

A difference in image resolution is not an obstacle to an OVD in whichone group of the image elements are diffuse pixels for creating agrayscale image and another group of the image elements are RGB pixelsfor a color image, possibly the same image as the grayscale.

Production Techniques

FIG. 9 shows a cross sectional view of an optical security device 70incorporating an eight-channel OVD structure. Optical security device 70includes a layer of embossable radiation curable ink applied to atransparent or windowed area of substrate 71 prior to being embossed,while still soft, to form refractive lenticular focusing structures72-74 on one side 75 of substrate 71. Each focusing structure 72-74 mayinclude a refractive cylindrical lens.

The ink may be cured by radiation to fix the embossed lenticularfocusing structures 72-74. Each focusing structure 72-74 is formed suchthat its focal length is approximately equal to the distance to theopposite side 76 of substrate 71. In some instances, it may be desirableto have lens which are do not focus exactly on the image plane (seeWO2010099571, which is hereby incorporated by reference) Each focusingstructure 72-74 facilitates detecting image elements 77-79 located onthe opposite side 76 of substrate 71.

The image elements 77-79 may be formed by radiation curable ink, throughembossing into a layer of such ink, or by printing of the radiationcurable ink in the desired pattern.

It is noted that the above method is the preferred method of creatingsuitable lenses and image elements but other methods may also beappropriate.

The above arrangement may produce an optical effect similar tolenticular based optical variable devices and may be visible to the eyethrough lenticular structures 72-74 located to detect diffractive imageelements 77-79 on the opposite side 76 of substrate 71. Each imageelement 77-79 includes an eight-channel OVD structure 60 comprisingimage pixels 61-68 (strips) as described above with reference to FIG. 6.Each set of image pixels 61-68 belongs to a distinct image or channel,so that as an observer viewing the device changes the angle of view, adifferent image or channel becomes visible. Each image or channel mayrepresent a full color portrait from one of eight different viewpoints.The net impression on a viewer emerging from viewing eight differentviewpoints is to generate a stereogram of the image with an addedbenefit of separation via lenticular focusing structures 72-74.

FIG. 10 shows a cross sectional view of an optical security device 80incorporating an eight-channel OVD structure. Optical security device 80includes a layer of embossable radiation curable ink applied to atransparent or windowed area of substrate 81 prior to being embossedwhile soft to form diffractive lenticular focusing structures 82-84 onone side 85 of substrate 81. Each focusing structure 82-84 may include adiffractive cylindrical lens or zone plate.

The ink may be cured by radiation to fix the embossed lenticularfocusing structures 82-84. Each focusing structure 82-84 may be formedsuch that its focal length is equal to the distance to the opposite side86 of substrate 81. Each focusing structure 82-84 may facilitatedetecting image elements 87-89 located on the opposite side 86 ofsubstrate 81. Image elements 87-99 can be formed in the same manner asdescribed in relation to FIG. 9. Each image element 87-89 may include aneight-channel OVD structure 60 as described above. The latterarrangement may produce a stereogram effect similar to that describedwith reference to FIG. 9.

FIG. 11 shows a flow diagram of a method of creating the structuresaccording to at least one aspect of the invention. Particularly, thestructures required for both lenses and image elements are formed byelectroforming or by nanoimprint microlithography in step 92. Thestructures created by the process in step 92 are then used, aftercreation into appropriate plates, or shims, to, preferably, emboss thestructures into an appropriate medium, such as a radiation curable ink,in step 94. This can be done in the same in-line system in a methodknown as “Double Soft Emboss Technology” or DSET, or other similartechnologies, such as that disclosed in WO2014070079 by Rolling OpticsAB, which is hereby incorporated by reference. Alternatively, thestructures for the lenses and image elements may be created separatelyand laminated together to form the final OVD.

As previously discussed above in the Summary of Invention section, theimage may include a portrait of an object such as a human face andgroups of image elements or channels may represent the object from manydifferent viewpoints. Projectional views of the object may be capturedso that the final stereogram produces an accurate three-dimensionalimage of that object. One technique for capturing stereograms isdescribed in the previously referenced MIT paper entitled “TheGeneralized Holographic Stereogram” by Michael W. Halle:

-   -   http://www.media.mit.edu/spi/SPIPapers/halazar/halle91.pdf.

A simple stereogram model may consist of a single holographic plateincluding a series of thin vertical slit holograms exposed one next tothe other across the plate. Each slit may be individually exposed to animage projected onto a rear-projection screen some distance away fromthe plate. Once the hologram is developed, each slit may form anaperture through which the image of the projection screen at the time ofthat slit's exposure may be seen. The images projected onto the screenmay include views of a scene or an object captured from many differentviewpoints.

Alternatively, a type of stereogram described in the prior art issimilarly created from multiple two dimensional pictures of an object orscene, but taken along a horizontal path to capture multiple horizontalviewpoints of the object or scene. In this type of stereogram, theimages thus captured are projected onto a holographic plate from thedirection in which they were originally taken. In one method, a plate isexposed multiple times with a constant reference to the images capturedabove, sequentially. Another way to achieve an identical result is tocreate an intermediate hologram which contains the individualperspective images separated left to right. This intermediate hologramis then used as a master to project all the images back to a plate wherethe final hologram is created.

Of the above types of stereograms, the most appropriate for thisinvention is a variation of the method of individually capturing images.To create the image elements in groups which will be viewed through thefocussing elements, images taken from different perspectives can befocused onto and projected through a series of slits onto a holographicplate. These slits must be spaced on the same centers as the focussingelements, and with a width equal to the individual focussing elementwidth divided by the number of images captured or at any rate projected.

A simpler method to produce the frames appropriate for the groups ofimage elements described herein, is to digitally interleave the imagescaptured above, resulting in an image element array identical to the onedescribed immediately above. The above arrangement may address theshortcomings of prior art OVDs by relying on focussing elements toseparate horizontal image channels. In particular the imaging propertiesof the focussing elements may not be dependent on the angle of incidenceof illuminating light; rather the clarity of the image may be determinedby the spatial relationship of the image elements to the focusingelements, which relationship may be fixed after embossing and may beindependent of illumination.

FIG. 12 shows a flow diagram of a method of generating the imageelements according to at least one aspect of the invention. Firstly,appropriate images are captured in step 102, as discussed in more detailbelow, as a series of frames. Secondly, the images are processed to besuitable for the image elements of the invention in step 104. Thisinvolves, in software:

-   -   1. Placing the frames to a set depth    -   2. Slicing into channels, according to lens widths and number of        frames    -   3. Reassembling the image into a single interlaced image    -   4. Compressing in the y-axis    -   5. Splitting the channels into RGB    -   6. Splitting the channels into horizontal rows to create        greyscale values which will be reassembled as color in the OVD

Then, in step 106, the digital output of step 104, being a matrix ofimage elements, is originated to form a structure, such as in a plate orshim, which can be used to replicate the image elements for the OVD.

FIG. 13 shows three methods of capturing the collection of twodimensional images, or frames, taken from different perspectives thatare required for holographic stereograms. One method 112 includesplacing an object or person 112B on a turntable 112A and film it with avideo or film camera 112C as it rotates. Images of the object or person112B are captured from multiple perspectives 112D. Another method 114includes placing a rail 114A in front of the scene/object/person 111Band driving a camera 114C along the rail 114A while it is recordingvideo, film, or taking a series of images from different perspectives114D. Another method 116 includes affixing a number of cameras 116C tothe rail 116A in the configuration of method 114 and triggering all thecameras 116C simultaneously to capture images of the object 116B fromdifferent perspectives 116D.

As shown in FIG. 14, Each of these techniques may record a series ofimages taken from different perspectives (1, 2 . . . n), and whenrecombined holographically or multiplexed 118 for a lenticularstereogram, replay the perspective images from their original angles,allowing the viewer 120 to perceive the scene/object/person as threedimensional. The viewer's eyes 122, 124 may see two images fromdifferent perspectives 126, 128 because the images are replayed towardthe original angles they were recorded from.

In holographic stereograms, the various perspective images are separatedby being diffracted to the original angle from which they were recordedwith the camera. Single and two step methods which have been used toachieve this end are well described in the literature.

One of the weaknesses of this type of holographic stereogram is thatwhen an extended light source, e.g. fluorescent lighting, officelighting, overcast sky, is used to illuminate the hologram, the variousperspectives become mixed together and the image becomes blurry andindistinct, at least any portions of the (3D) image which do not lie inthe plane of the hologram.

In a 3D lenticular display, the various perspective views captured aboveare multiplexed together into an image matrix of interleaved verticalchannels which are then printed with typical printing inks. An array ofvertical cylindrical lenticules is then placed at roughly the focallength of the lenticules from this image matrix, and all the imageschannels associated with a particular angle are refracted in the samedirection from the device, a direction which is unique for eachperspective image. The multiplexed array may be arranged so that theimages are in the correct order. For example, if only 4 perspectiveimages were to be used, the array may be vertical slices of each imagein the order 1,2,3,4,1,2,3,4,1,2,3,4,1,2,3,4 . . . etc. and each set offour channels may lie under one lenticule.

A number of methods for creating the OVD of the present inventiondescribed above exist. In all embodiments, a series of perspectiveimages as described above are obtained. In the preferred embodiment,those perspective images are aligned left to right in such a way thatduring reconstruction the 3D image is created at a predetermined depthrelative to the device. As an example, a point on an object is selectedto be on the surface. All the perspective images are shifted and/orcropped so that this point is in the same horizontal position in eachimage frame. Then the images are divided into columns, and interlacedinto a final image file that consists of all the images arranged, forexample, above, 1,2,3,4,1,2,3,4,1,2,3,4,1,2,3,4 . . . numbered byperspective number. However, and to be clear, each column remains in theoriginal position of the original perspective image (after thehorizontal shift to set the image plane). For example, if the columnsare numbered by the horizontal position in whichever file they wereoriginally part of, the numbering would be:1,2,3,4,5,6,7,8,9,19,10,11,12,13,14,15 . . .

Another less desirable and lower resolution interlacing techniqueyielding a dimensionally equivalent result to the above example (with 4image perspectives) would be to use columns1,1,1,1,4,4,4,4,8,8,8,8,12,12,12,12 . . .

One of the weaknesses of 3D lenticular displays is that the minimumwidth of the multiplexed slits is related to the resolution achievableby the printer, which forces the use of fewer images and/or largerlenticules than are ideal. However, these displays are not degraded byextended source lighting, and are equally sharp in diffuse and pointsource lighting.

The OVD device of the present invention addresses the disadvantages ofboth these 3D display technologies. Holography can record image elementsthat are on the order of a few wavelengths of light, therefore extremelynarrow channels can be used in the device. This may allow the use ofmany more perspective images or frames than in a similar printeddisplay: it may also allow the use of smaller lenticules, whichincreases the overall resolution of the image and makes the devicethinner, easier to produce, and less costly. Because the 2D perspectiveimages are multiplexed in the same way as they are in a printed 3Dlenticular image, and because the lenticules redirect these images intothe correct angles, the horizontal separation of the variousperspectives is not affected by lighting and the image remains sharpunder less than ideal lighting.

The embodiments of FIGS. 9 and 10 may produce a primary optical effectthrough focusing structures 72-74, 82-84 which focus on the back 76, 86of substrates 71, 81 as described above but may also produce a secondaryoptical effect when image elements 77-79, 87-89 are viewed from the backof substrates 71, 81. Applicant believes that the secondary effect iscaused by focusing structures 72-74, 82-84 providing surfaces whichproduce weak sampling by directing light in slightly greater preferenceback through the imagery. Moving the OVD device may change that point ofpreference in the imagery and cause the eye to see the OVD effect. Somelight may be due to total internal reflection and some may be due toweak reflection that occurs from outer surfaces of focusing structures72-74, 82-84. A protective coating, such as a transparent varnish may beapplied over lenticular focusing structures 72-74, 82-84. The protectivecoating may be applied over the lenticular focusing structures 72-74,82-84 as well as over other areas of substrate 71, 81 in which focusingstructures 72-74, 82-84 are not present. The latter areas (not shown)may contain a 2D image that may be visible at least when the device isrotated about an axis perpendicular to the plane of the device or isrocked back and forth about an axis within the plane of the device. The2D image may comprise a diffractive or non-diffractive structure such asa color changing ink.

Preferably, the protective coating includes a high refractive index(HRI) coating, as this may assist in ensuring that the optical effectproduced by lenticular focusing structures 72-74, 82-84 remains visibleeven if the coating is applied in a thick layer which does not followthe contours of the lenses. However, in other embodiments possiblecoatings may include a transparent, non-high refractive varnish.

In a similar manner to the focussing structures, coatings may also beapplied over the image elements 77-79, 87-89.

It may be appreciated that a suitable coating should demonstrate one orall of the following attributes including: good adhesion to thesubstrate, highly transparent, generally colorless, and robustness.Possible coatings may include a transparent, non-high refractivevarnish. Varnish may denote a material that results in a relativelydurable and protective finish. Exemplary transparent varnishes mayinclude, but are not limited to, nitrocellulose and cellulose acetylbutyrate. Alternatively, the coating may include a high refractive indexcoating, being a coating having a metal oxide component of smallparticle size and high refractive index dispersed in a carrier, binderor resin. Such a high refractive index coating may contain solvent as itis a dispersion. Where a high refractive index coating of this type isused, it may be air cured or UV cured.

Alternatively, a high refractive index coating utilising a non-metallicpolymer, such as Sulphur-containing or brominated organic polymers mayalso be used.

Advantages of the optical security device of the present invention mayinclude:

-   -   1. Real 3D imagery may be mass produced with increased depth and        clarity in a relatively inexpensive form that may be immediately        integrated into existing banknote technology.    -   2. Detail provided by high resolution diffractive imagery may be        much greater than is currently possible with high throughput        print technologies.    -   3. 3D imagery may be easily differentiated from 2D imagery by a        lay person. This and difficulties in counterfeiting or        simulating such 3D imagery should result in improved security.

4. Grayscale 3D imagery generated from OVD's made using cost effectivehigh volume fabrication techniques can be viewed from a broad range ofviewing angles and with greater image resolution than color images inthe RGB color space.

-   -   5. Further security hurdles for any counterfeiter may include:        -   a) Obtaining first stage photographic imagery of a person or            place;        -   b) Developing OVD micro-technology to duplicate or simulate            stereographic diffractive channels;        -   c) Developing specific cylindrical microlenses required for            the above;        -   d) Registering microlenses accurately to stereographic            imagery on a polymer substrate;        -   e) Since portraiture is often used in banknote design, the            image used may capture a specific person at a specific            moment in time. This may significantly increase difficulty            of mimicking a portrait.    -   6. Marriage of two previously separate 3D technologies, namely,        microscopic lenticulars and diffractive image elements may        facilitate creation of a product not possible up to now, namely        a clear 3D portrait (or other image) on a bank note.

Finally, it is to be understood that various alterations, modificationsand/or additions may be introduced into the constructions andarrangements of parts previously described without departing from thespirit or ambit of the invention.

1. A security device including: a plurality of focusing elements; aplurality of image elements associated with each focusing elementwherein said image elements include at least a first and a second groupof image elements, wherein each image element is located in an objectplane to be viewable through the associated focusing element, whereineach image element comprises a diffractive grating element orsub-wavelength grating element which when illuminated by a light sourcegenerates a diffraction image observable at a range of viewing anglesaround the device, and image elements of the first group are visible ina first range of viewing angles and image elements of the second groupare visible in a second range of viewing angles.
 2. A security deviceaccording to claim 1, wherein said image elements include three or moregroups of image elements to represent an image observable from differentviewing angles.
 3. A security device according to claim 1 wherein thediffraction image is a greyscale or monochromatic image that includes aplurality of brightness levels across the image, and/or a color imagethat includes a plurality of colors.
 4. A security device according toclaim 1, wherein each image element comprises red, green and bluesub-elements, and the red, green and blue sub-elements each include itsown diffractive grating element or sub-wavelength grating element,wherein the frequency and/or the pitch of the diffractive gratingelements or sub-wavelength grating elements are different in the red,green and blue sub-elements so that each sub-element produces apredetermined primary color upon illumination.
 5. (canceled)
 6. Asecurity device according to claim 4, wherein the red, green and bluesub-elements are vertically arranged as a strip, within the redsub-element being located at the top of the vertical strip, the greensub-element being located in the middle, and the blue sub-element beinglocated at the bottom of the strip.
 7. (canceled)
 8. A security deviceaccording to claim 6, wherein the red, green and blue sub-elements areof a same physical size, with the grating elements of the red, green andblue sub-elements comprising a size distribution and/or a spatialdistribution corresponding to grey levels or brightness levelsassociated with the sub-element.
 9. (canceled)
 10. A security deviceaccording to claim 4, wherein each of the sub-elements includes aneffective grating area that includes the diffraction grating element orthe sub-wavelength grating element, and a non-diffractive area that doesnot include any grating elements, and a brightness value of eachsub-element is varied by changing the effective grating area within eachsub-element and/or the non-diffractive area of the sub-element.
 11. Asecurity device according to claim 10, wherein the non-diffractive areawithin each of the red, green and blue sub-elements are of a same sizefor a given image element, to thereby generate a greyscale image, orthat the non-diffractive area within each of the red, green and bluesub-elements are of a different size for a given image element, tothereby generate a color image.
 12. A security device according to claim1, wherein each image element includes a randomized diffraction gratingwith a randomized grating pitch and/or width to diffract incident lightat different angles, such that incident light diffracted from thediffraction grating is diffused and the diffraction image observable isa greyscale image.
 13. A security device according to claim 12, whereineach image element includes an effective grating area that includes thediffraction grating element or the sub wavelength grating element, and anon-diffractive area that does not include any grating element, and abrightness value of each image element is varied by changing theeffective grating area and/or the non-diffractive area of the imageelement.
 14. A security device according to claim 10, wherein thenon-diffractive area includes a microstructured surface with light trapsfor creating internal reflections of most incident light that then failsto reflect out and away from the light traps; or the non-diffractivearea has a reflective coating for specular reflection of incident light;or the non-diffractive area has a light absorbing coating. 15.-18.(canceled)
 19. A security device according to claim 1, wherein thediffractive grating element or sub-wavelength grating element is formedfrom a surface relief structure formed in a radiation curable ink, andwherein the focusing elements are formed from a radiation curable ink byprinting and/or embossing. 20.-21. (canceled)
 22. A security deviceaccording to claim 1, wherein the focusing elements include refractiveor diffractive part-cylindrical lenses or zone plates.
 23. (canceled)24. A security device including: a plurality of focusing elements; aplurality of image elements located in an object plane to be viewablethrough the focusing elements, said image elements including at leastfirst and second groups of image elements, wherein each image elementincludes a surface structured for causing diffuse scattering of incidentlight, wherein said plurality of image elements are arranged to generatean image observable when illuminated by the incident light, and imageelements of the first group are observable in a first range of viewingangles and image elements of the second group are observable in a secondrange of viewing angles.
 25. A security device according to claim 24,wherein the surface structured for causing diffuse scattering ofincident light includes a randomized diffraction grating with a randomgrating pitch to diffract light of a given wavelength at differentangles such that white incident light diffracted from the randomdiffractive grating is diffused and the image observable is a greyscaleimage.
 26. A security device according to claim 24, wherein the surfacestructured for causing diffuse scattering of incident light may includean array of randomly arranged micromirrors which cause incident light tobe diffuse scattered in different directions.
 27. A security deviceaccording to claim 25, wherein each image element includes an effectivegrating area that includes the randomized diffraction grating element,and a non-diffractive area that does not include any grating element,and a brightness value of each image element is varied by changing theeffective grating area and/or the non-diffractive area of the imageelement.
 28. A security device according to claim 27, wherein thenon-diffractive area includes any one or more of: a microstructuredsurface with light traps for creating internal reflections of mostincident light that then fails to reflect out and away from the lighttraps, a reflective coating for specular reflection of incident light,and a light absorbing coating.
 29. A security device according claim 1,wherein the image elements are anamorphic, and more preferably the imageelements are configured to have a rectangular shape, such that thelengths of the image elements are greater than the widths. 30.(canceled)
 31. A security device according to claim 1, wherein the imageobservable is a three dimensional image of a scene, object and/or aperson. 32.-37. (canceled)